Material: Made of plastic and metal material,fine workmanship,good performance,durable,convenient. Working Principle: Through connect continuous automobile battery voltage(11V-15V) to the relay coil, the detector can properly check the relay contact function. LED Indicator Light: If red LED will light,the relay is abnormal.If green LED will light,the relay is normal. Design: Compact design,light weight,small size,portable,easy to carry and use, work accurately,perfect choice for you. Applications: Test to properly control the opening and closing of the relay. Specification: Material: plastic and metal,fine workmanship,durable. Color: black and red Voltage: 12V Size: 11x 6x2.8cm/4.3x2.4x1.1inch(Due to manual measurement, there will be an error of 1-3mm) Feature: Working Principle: Through connect continuous automobile battery voltage (11V-15V) to the relay coil, the detector can properly check the relay contact function. LED Indicator Light: If red LED will light,the relay is abnormal.If green LED will light,the relay is normal.It's easy to detect, so you can know what's wrong. Applications: Test to properly control the opening and closing of the relay. A test tool to automotive relays. Design: Compact design,light weight,small size,work accurately,perfect choice for you. The tester observes contact data, closure time, relay inertia at each test stage. Package List: 1 x relay tester Note: Car battery must be 12V battery, the voltage must be between 11-15V. The coil resistance is preferably at least 20 ohms.